Lining systems for containment systems (e.g., systems which contain bodies of water such as ponds) and the like are used to provide an “impermeable” barrier between contaminants and the underlying ground. Generally, these liners are made of insulating material (such as high density polyethylene) which, even if thoroughly tested to be defect free when shipped, can be damaged during shipping and/or installation by, for example, heavy equipment, cutting tools, welding equipment, animals, and vandalism, necessitating that a final leak check be conducted after the liner is installed to locate leaks caused by any such damage. The liner can also be damaged after it is covered by soil and/or liquid, including during its service life as a result, for example, of stones, rocks and/or settlement. Detecting such leaks is important, particularly where hazardous materials are involved, as holes as small as 1.0 millimeter in diameter may cause leaks on the order of a couple of gallons per day with one foot of water pressure.
Electrical leak location has heretofore been used which involves placing an electrical potential across a geomembrane and then locate the points of anomalous potential distribution where electrical current flows through leaks in the geomembrane. The electrical potential is typically applied utilizing a power supply with the positive electrode submerged in water or a soil layer above the geomembrane, and the negative electrode connected to the soil layer below. When there are leaks, electrical current flows through the leaks, which produces high current density and a localized anomaly in the potential distribution in the material above the geomembrane. Electrical measurements are made to locate those areas of anomalous signal at the leaks. ASTM D7002 and D7007, for example, include details pertaining to such tests. Such measurements have been made using a dipole or pole measurement configuration (though various types of data acquisition equipment can be used), with point by point measurements commonly made using either dipole or pole measurements along parallel lines on a grid pattern.
In one such method of electrically detecting liner leaks, for example, a potential is induced across the thickness of a liner. If a potential of one polarity is induced on one side of the sheet and a potential of the opposite polarity is induced on the opposite side of the sheet, the resulting electrical field will be affected if there is any conductivity from side to side across the sheet, with the effects on the conduction monitored to detect the presence of a leak. Such a detecting method requires an electrically conductive media both above and below the liner, which can be provided by liquid or soil above the liner and good electrical contact with a conductive underlying soil.
However, in some installations, electrically detecting leaks in the above described manner is unreliable. For example, if the liner is not maintained in good electrical contact with the earth (due to, e.g., use of double liners or other insulating materials, irregularities in the subgrade, and/or wrinkles in the liner) and/or the earth under the geomembrane is dry or not conductive or highly resistant (e.g., in a landfill or with a mining heap leach pad, secondary containment, or coal ash containment), reliable measurements of potential may not be obtained. Similarly, in some landfills, there is leak detection layer of either sand, gravel or geosynthetic product directly underneath the geomembrane for draining any leakage through the geomembrane to a detection site, which layer can inhibit or nullify the leak location survey due to the lack of conductivity of the material.
One solution to this unreliability arising from possibly insufficient electrical conductivity on the underside of the liner was suggested in U.S. Pat. No. 3,252,155, which disclosed placing the liner over or even adhesively secured to a metal foil sheet, where the foil would provide the required underlying conductivity. That technique was not widely accepted in the industry, however, as such foil is expensive, securing the metal foil to the liner, whether adhesively or mechanically, is extremely difficult to achieve, and the exposed metal foil could severely degrade as a result of, for example, galvanic corrosion, at the construction site.
Spencer U.S. Pat. No. 5,288,168 (the full disclosure of which is hereby incorporated by reference) has significantly improved upon the foil sheet suggestion by disclosing a liner having an electrically conductive layer provided by embedding conductive particles in the bottom of the layer. The integrity of the sheet is then monitored by establishing an electric field across the sheet and monitoring for sparks between a probe and the bottom, conductive plastic layer. Such spark testing has been accomplished, for example, with a test device that includes a high voltage power source with the positive lead attached to a brass brush and the negative lead attached to a conductive neoprene grounding pad laid on top of the geomembrane. See, for example, ASTM 7240.
Spark testing of seams in particular has heretofore been done such as detailed in ASTM D6365, wherein conductive material is inserted into the seam just prior to or during fabrication of the seam, with the conductive material connected to a negative terminal of a test apparatus and a positive voltage applied across the seam edge such that a suspect area in the seam is indicated by a spark from the voltage source to the conductive material.
While the Spencer '168 invention significantly improved leak detection in testing panels, it should be appreciated that during construction of a lined pond, leaks may be caused in a geomembrane panel which was found by testing to have no leaks immediately after liner installation (e.g., by puncturing a liner when it is covered in place by soil and/or water). Moreover, since such lined facilities are typically constructed using a plurality of geomembrane panels heat welded together along seams, testing of the individual panels will not detect leaks at the seams of the panels, where false and anomalous readings have been found. Still further, the conductivity of individual liner panels is often still insufficient for reliable testing, particularly where the liner panel is not maintained in good electrical contact with the earth (due to, e.g., use of double liners, irregularities in the subgrade, and/or wrinkles in the liner) and/or the earth is dry or not conductive.